1. Field of the Invention
This invention relates to an assembly method for delivering and deploying an inflation expandable stent, particularly within a lumen of a body vessel. More specifically, this invention relates to the provision of a securement component positioned over the inner catheter, and beneath a balloon and a loaded stent, to maintain the stent on the catheter assembly during delivery to a stent deployment site.
2. Description of Relevant Art
Stents and stent delivery assemblies are utilized in a number of medical procedures and situations, and as such their structure and function are well-known. A stent is a general cylindrical prosthesis introduced via a catheter into a lumen of a body vessel in a configuration having a generally reduced diameter and then expanded to the diameter of the vessel. In its expanded configuration, the stent supports and reinforces the vessel walls while maintaining the vessel in an open, unobstructed condition.
Both self-expanding and inflation expandable stents are well-known and widely available. Self-expanding stents must be maintained under positive external pressure in order to maintain their reduced diameter configuration during delivery of the stent to its deployment site. Inflation expandable stents (also known as balloon expandable stents) are crimped to their reduced diameter about the delivery catheter, positioned at the deployment site, and then expanded to the vessel by diameter by fluid inflation of the balloon positioned between the stent and the delivery catheter. The present invention is particularly concerned with enhanced stent securement and safer stent loading in the delivery and deployment of balloon expandable stents.
In angioplasty procedure, there may be restenosis of the artery, which either necessitates another angioplasty procedure, a surgical bi-pass procedure, or some method of repairing or strengthening the area. To prevent restenosis and strengthen the area, a physician can implant an intravascular prosthesis for maintaining vascular patency, i.e. a stent, inside the artery at the lesion. The stent is expanded to a larger diameter for placement in the vasculature, often by the balloon portion of the catheter. Stents delivered to a restricted coronary artery, expanded to a larger diameter as by a balloon catheter, and left in place in the artery at the site of a dilated lesion are shown in U.S. Pat. Nos. 4,740,207 to Kreamer; 5,007,926 to Derbyshire; 4,733,665 to Palmaz; 5,026,377 to Burton et al.; 5,158,548 to Lau et al.; 5,242,399 to Lau et al.; 5,344,426 to Lau et al.; 5,415,664 to Pinchuck; 5,453,090 to Martinez et al.; 4,950,227 to Savin; 5,403,341 to Solar; 5,108,416 to Ryan et al.; and European Patent Application No. 707837A1 to Scheiban, all of which are incorporated herein by reference. A stent particularly preferred for use with this invention is described in PCT Application No. 96/03092-A1, published Feb. 8, 1996, the content of which is incorporated herein by reference.
In advancing a balloon expandable stent through a body vessel to the deployment site, there are a number of important considerations. The stent must be able to securely maintain its axial position on the delivery catheter. The stent, particularly its distal and proximal ends, are sometimes protected to prevent distortion of the stent, and minimize trauma to the vessel walls. Balloon expandable stent delivery and deployment assemblies are known which utilize restraining means that overlay the stent during delivery. U.S. Pat. No. 4,950,227 to Savin et al., relates to a balloon expandable stent delivery system in which a sleeve overlaps the distal or proximal margin (or both) of the stent during delivery. During inflation of the stent at the deployment site, the stent margins are freed of the protective sleeve(s) and the sleeves then collapse toward the delivery catheter for removal. A number of balloon expandable stent delivery and deployment assemblies do not use overlaying restraining members, such as the Savin sleeves, to position the stent for delivery. European Patent Application No. EP 055 3960A1 to Lau et al., uses an elastic sheath interspaced between the balloon and the stent. The sheath is said to act as a barrier to protect the balloon from the stent, allow uniform stent expansion, decrease balloon deflation time, prevent undesirable balloon flattening upon deflation and provide a friction substrate for the stent. The Lau sheath can be positioned on the inside or outside of the balloon. U.S. Pat. No. 5,409,495 to Osborne, similarly uses an elastic sleeve or sheath surrounding and in contact with the balloon for controlling the balloon radial expansion. In addition, Osborne is said to use restraining bands or a pair of balloons to achieve controllable stent expansion characteristics. U.S. Pat. No. 5,403,341 to Solar, relates to stent delivery and deployment assembly which uses a retaining sheath positioned about opposite ends of the compressed state. The retaining sheaths of Solar are adapted to tear under pressure as the stent is radially expanded, thus releasing the stent for engagement with the sheaths. U.S. Pat. No. 5,108,416 to Ryan et al. describes a stent introducer system which uses one or two flexible end caps and annular socket surrounding the balloon to position the stent during introduction to the deployment site. The content of all of these patents is incorporated herein by reference.
In positioning a balloon expandable stent on the delivery catheter over the fluid expandable balloon, the stent must be smoothly and evenly crimped to closely conform to the overall profile of the catheter and the unexpanded balloon. It has been noted that, due to physical properties of the material used in manufacturing the stent (stainless steel, tantalum, platinum or platinum alloys, or shape memory alloys such as Nitinol.TM.) there is a certain amount of "recoil" of the stent despite the most careful and firm crimping. That is the stent evidences a tendency to slightly open up from the fully crimped position and once the crimping force has been released. For example, in the typical stent delivery and deployment assembly, if the stent has been fully crimped to a diameter of approximately 0.0035", the stent has been observed to open up or recoil to approximately 0.0037". This phenomenon has been characterized as "recoil crimping". Due to recoil crimping to this slightly enlarged diameter, it can be understood that the stent tends to evidence a certain amount of looseness from its desired close adherence to the overall profile of the underlying catheter and balloon. That is, the stent tends to have a perceptible relatively slack fit in its mounted and crimped position. During delivery, the stent can thus tend to slip and dislocate from its desired position on the catheter or even become separate from the catheter, requiring further intervention by the physician.
Further, there is a possibility of damaging the balloon during the stent crimping as a result of pinching the balloon material between the metal stent and any metal (or protruding object) on the inner guide lumen (e.g. marker bands).
According to the present invention, a securement means such as a corrugated (accordion-type) tube is secured over the inner catheter beneath the balloon to compensate for the undesired looseness or slack that due to recoil crimping and to aid in securing the stent to the balloon, as well as protecting the balloon material from being sandwiched between the stent and any metal or protruding item which may be mounted on the inner shaft/guide wire lumen, for delivery of the stent. The corrugated tube provides additional volume for improved stent securement, i.e. more surface area to crimp onto, and also maintains flexibility. In addition, when metal marker bands are employed on the inner catheter, the tubing aids in preventing damage to the balloon during crimping/loading of the stent. The tubing, which may be inflatable, compensates for the perceptible looseness due to recoil crimping and secures the stent during tracking and delivery and provides a good friction fit to the stent and insures good contact between the stent and underlying balloon and catheter, instead of merely relying on the bulk of the flaccid balloon over the underlying catheter to hold the stent on. According to the present invention, the tubing component will compensate for slackness in the fit of the stent due to recoil crimping.
According to another embodiment of the present invention, the securement means is an expandable tube component positioned under the expandable balloon to compensate for this undesired looseness or slack fit due to recoil crimping and to aid in securing the stent to the balloon and the catheter for delivery. The expandable tube component and the expandable balloon are each provided with separate, individually controllable fluid inflation sources. Once the stent has been fully crimped to conform to the overall profile of the catheter, the expandable balloon, and the underlying expandable tube component, the tube component is inflated. The tube component is inflated to at least the limits of the elastic deformation of the fully crimped stent. It is desirable to slightly further inflate the tube component to a pressure at which the fully crimped stent just begins to plastically deform. That is, the tube component may be inflated to a point at which the stent is just barely beginning to provide resistance to the expansion of the tube component, which is also characterized as a point at which the stent just barely begins to expand beyond the crimped position (taking into consideration the recoil crimping phenomenon). The desired pressure to which the tube component is inflated is characterized as the "securement pressure". The application of securement pressure to the tube component compensates for the perceptible looseness due to recoil crimping and secures the stent during tracking and delivery. The application of securement pressure to the tube component provides a good friction fit to the stent and ensures good contact between the stent and the underlying balloon, "securement pressurized" tube component and catheter. The desired diameter of the stent upon the application of securement pressure to the tube component is characterized as the "delivery diameter", because in this condition the stent can safely, reliably and securely be delivered to the pre-selected position within a body vessel. Instead of merely crimping the stent onto the balloon and the underlying catheter and relying on the bulk of the flaccid balloon to hold the stent on, according to the present invention, the expandable tube component will compensate for slackness in the fit of the stent due to recoil crimping. Prior to inflation of the tube component to the securement pressure, the physician preparing the assembly may manually sense a looseness of the stent in its position. When the tube component has been inflated to the necessary securement pressure, the physician will manually sense that the stent is securely retained or "stuck" in position. The expandable tube component is designed and constructed to be expandable to no more than is necessary to compensate for recoil crimping and is incapable of overexpanding to provide the pressure needed to fully expand the stent to its deployment position.
There are a number of descriptions of catheters which use a pair of coaxial, at least partially coextensive balloons. U.S. Pat. No. 5,512,051 to Want et al., describes a slip layered balloon made of a plurality of layers with a low friction substance between the layers. During expansion the layers are able to slide relative to each other softening the balloon while maintaining its strength. U.S. Pat. No. 5,447,497 to Sogard et al., relates to s a dual layered balloon, in which one balloon is compliant and the other is non-compliant, so that the balloon assembly has a non-linear compliance curve. U.S. Pat. No. 5,358,487 to Miller, describes a balloon catheter having an outer balloon surrounding an inner balloon. The inner balloon has a maximum inflation diameter less than that of the outer balloon, so that, upon inflation, the inner balloon bursts at a certain diameter, allowing the outer balloon to be further expanded. U.S. Pat. No. 5,290,306 to Totta et al., relates to a balloon catheter with an outer elastomeric sleeve to provide the balloon with pin hole and abrasion resistance. U.S. Pat. No. 5,049,132 to Shaffer et al., describes a first balloon and a second balloon, each having separate inflation lumens, the second balloon having apertures for controlled administration of a medication therethrough. U.S. Pat. Nos. 4,608,984 and 4,338,942 to Fogarty, each relate to a catheter with an inner non-elastic balloon and an outer elastic balloon. In each patent, the outer balloon aids in collapse and retraction of the inner balloon. U.S. Pat. No. 4,328,056 to Snooks, describes a method of making a double cuffed endotracheal tube component. However, none of these references show, suggest or render obvious an inner balloon, in conjunction with an outer deployment inflatable balloon, to provide securement pressure to a compressed stent during delivery to a site within a body vessel.
The art referred to and/or described above is not intended to constitute an admission that any patent, publication or other information referred to herein is "prior art" with respect to this invention. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. .sctn.1.56(a) exists.